This invention relates to a multipath noise reducer, an audio output circuit including a multipath noise reducer, and a frequency-modulation (FM) radio receiver including a multipath noise reducer.
Radio receivers are afflicted by various types of electromagnetic noise. Radio broadcast receivers mounted in automobiles, for example, must contend with ignition noise and mirror noise, which are impulsive in character and are generally referred to as impulse noise. These so-called car radios also experience episodes of multipath noise due to reflection of radio waves from hills, high buildings, and other passing objects. Multipath noise occurs because the car radio antenna receives both a line-of-sight signal, coming directly from the transmitting antenna, and reflected signals, reflected from the passing objects. The reflected signals tend to be out of phase with the line-of-sight signal, causing the line-of-sight signal to be partly attenuated by the reflected signals. The resulting deterioration in quality of the audio output from a car radio is a familiar experience to automobile riders.
Various methods of reducing noise are known. In an FM stereo car radio, one method is to detect the strength of the electric field received at the antenna, and take noise countermeasures when the field is weak. One countermeasure is to reduce the degree of stereo separation, or to switch completely from stereo to monaural operation. This countermeasure will be referred to below as stereo separation control. Another countermeasure is to attenuate or xe2x80x9ccutxe2x80x9d high-frequency components in the demodulated signal. This countermeasure will be referred to below as high-cut control. Both of these countermeasures improve the signal-to-noise (S/N) ratio during intervals when the electric field received at the antenna is weak.
To reduce impulse noise, car radios may also include an impulse noise reducer that detects the onset of impulse noise and generates a gate signal having a predetermined length sufficient to cover the expected duration of the impulse noise, which is typically only a fraction of a millisecond. When the gate signal is active, the signal output by the car radio is held constant, effectively suppressing the noise.
The gate pulse used in this type of impulse noise reducer is too short to mask multipath noise, the duration of which is typically much longer than the duration of impulse noise. The gate pulse could be lengthened to cover multipath noise intervals, but a long gate pulse would noticeably distort the audio output signal. Furthermore, the long gate pulse would be triggered by each short occurrence of impulse noise, resulting in much needless audio distortion during times when no noise was present.
Further details of these problems will be given in the detailed description of the invention.
An object of this invention is to reduce multipath noise with minimal output distortion.
The invented multipath noise reducer includes rectifying means, threshold-calculating means, comparator means, and modification means. The rectifying means generates a rectified signal corresponding to an absolute value of an input signal. The threshold-calculating means generates a threshold signal having a value related to the rectified signal. The comparator means detects multipath noise by comparing the rectified signal with the threshold signal, and outputs a detection signal. The modification means modifies the input signal according to the detection signal, thereby reducing multipath noise present in the input signal.
The active pulses in the detection signal do not have a predetermined width, but correspond to the actual width of multipath noise spikes occurring in the input signal. All multipath noise spikes can thereby be removed without distortion of other parts of the signal.
The rectifying means includes, for example, a high-pass filter and an absolute-value calculation means. The high-pass filter facilitates both the accurate detection of multipath noise by the comparator means, and the generation of an appropriate threshold signal by the threshold-calculating means.
The rectifying means may be configured so that the high-pass filter operates on the output of the absolute-value calculation means. This configuration enables the width of multipath noise spikes to be detected accurately with a comparatively simple high-pass filter.
The threshold-calculating means includes, for example, smoothing means for smoothing the rectified signal. The smoothing means prevents the threshold signal from being affected by signal variations below the noise floor level.
The threshold-calculating means may also include limiting means for limiting variations in the input and/or output of the smoothing means. Excessive variations in the threshold signal are thereby prevented.
The invented audio output circuit includes a multipath noise reducer as described above. The invented audio output circuit can accordingly generate an audio output signal that remains comparatively free of distortion even during intervals of multipath noise.
If the audio signal generated by the audio output circuit is a stereo signal, the audio output circuit may also include control means receiving the detection signal from the comparator means in the multipath noise reducer and generating therefrom a control signal controlling stereo separation of the audio signal. Distortion due to multipath noise can then be further reduced.
The control means comprises, for example, time-constant means for combining past and present values of the detection signal according to a predetermined time constant. Unwanted effects such as an unstable sound image and rapid volume fluctuations can then be avoided.
The control means may cause the control signal to vary in a stepwise manner, which also prevents rapid volume fluctuations.
The control means may cause control of the stereo separation of the audio signal to start at a zero-crossing point of the audio signal. Discontinuities in the output audio signal are thereby avoided.
The invention also provides an FM radio receiver including an audio output circuit as described above. FM broadcasts can then be received with reduced distortion due to multipath noise.
Alternatively, the invention provides an FM stereo radio receiver including an FM demodulator, a multipath noise reducer, and an impulse noise reducer. The multipath noise reducer reduces multipath noise in the demodulated signal output by the FM demodulator. The impulse noise reducer reduces impulse noise in the signal output from the multipath noise reducer. FM stereo broadcasts can then be received with reduced distortion due to both multipath noise and impulse noise.
The multipath noise reducer in this FM stereo radio receiver may of course be the invented multipath noise reducer, in which case the above-described effects of the invented multipath noise reducer are obtained.